Offshore platforms for the exploitation of undersea petroleum and natural gas deposits typically support risers that extend to the platform from one or more wellheads or structures on the seabed. In deep water applications, floating platforms (such as spars, tension leg platforms, extended draft platforms, and semi-submersible platforms) are typically used. These platforms are subject to motion due to wind, waves, and currents. The risers employed with such platforms must therefore be tensioned to permit the platform to move relative to the risers. Riser tension must also be maintained so that the riser does not buckle under its own weight. The tensioning mechanism must accordingly exert a substantially continuous tension force to the riser within a well-defined range.
Hydro-pneumatic tensioner systems are one form of riser tensioning mechanism typically used to support risers known as “Top Tensioned Risers” on various platforms. A plurality of passive hydraulic cylinders with pneumatic accumulators are thereby connected between the platform and the riser to provide and maintain the necessary riser tension. Platform responses to the above mentioned environmental conditions, mainly heave and horizontal motions, create changes in platform position relative to the riser, causing the tensioning cylinders to stroke in and out. The spring effect resulting from the gas compression or expansion during a platform's vertical and horizontal movement relative to the seabed partially isolates the riser from the platform's motions.
Riser tensioners constituting such hydro-pneumatic tensioning systems comprise cylinders in which the cylinder rods are at least indirectly connected to the riser so that the pressure induced movements of the rod relative to its support results in the desired riser tensioning. Such hydro-pneumatic tensioner systems are presently produced in a variety of dimensions and sizes. Examples of typical state of the art riser tensioner systems are disclosed in WO 2004/013452, U.S. Pat. Nos. 4,886,397, 3,902,319, GB 2.109.036, WO 2012/016765 and WO 2014/090682.
Known riser tensioning systems include so-called “Direct Acting Tensioner” (DAT) systems and so-called “Wireline Riser Tensioner” (WLT) systems. An example of such systems is described in WO 2012/016765 (for example, FIGS. 4 and 5).
The location of the DAT systems within a moon pool area of a floating platform or vessel results in the risk of exposure to seawater which introduces challenges related to the corrosive environment and heavy accelerations of cylinder and rod. DAT systems may also show poor performance due to internal load deviations within the system. There may, for example, be flow resistance in the pipework/hoses between the tensioning cylinders and accumulator(s).
The alternative WLT system is large and may require a fairly complex arrangement if the system is to be applied to drilling vessels with a setback/fingerboard (for storing drill pipes, drill collars, etc.) that is situated lower than the drill floor. Such a low setback arrangement is demanding, and most users choose DAT systems in these situations. In addition to the complex arrangement in drilling vessels with low setback, the WLT systems suffer from high load variations due to friction over the plurality of sheaves. The repeated bending of the wire over a number of sheaves increases wire wear. A cut and slip configuration or complete replacement of the wire may therefore be required. The WLT systems are often heavy compared to other riser tensioning systems, typically around 500 MT depending on capacity.